In a Long Term Evolution (LTE) system, a time-frequency resource is divided into an orthogonal frequency division multiplexing (OFDM) symbol or a single carrier frequency division multiple access (SC-FDMA) symbol (hereinafter referred to as a time-domain symbol) in a time-domain dimension and a subcarrier in a frequency-domain dimension. A minimum resource granularity is referred to as a resource element (RE), that is, a time-frequency grid including one time-domain symbol in a time domain and one subcarrier in a frequency domain. In the LTE system, service transmission is performed based on scheduling by a base station. A basic time unit of scheduling by the base station is a subframe. One subframe includes multiple time-domain symbols. A specific scheduling procedure is as follows: The base station sends a control channel, such as a physical downlink control channel (PDCCH) or an enhanced PDCCH (EPDCCH). The control channel may carry scheduling information of a data channel, such as a physical downlink shared channel (PDSCH) or a physical uplink shared channel (PUSCH), and the scheduling information includes control information, such as resource allocation information and a modulation and coding scheme. A user equipment (UE) detects the control channel in a subframe, and receives a downlink data channel or sends an uplink data channel according to the scheduling information carried on the detected control channel.
LTE supports frequency division duplex (FDD) and time division duplex (TDD). In an FDD system, uplink transmission and downlink transmission are performed on different carriers. In a TDD system, uplink transmission and downlink transmission are performed on a same carrier at different time.
LTE currently supports seven different TDD uplink-downlink subframe configurations. As shown in Table 1, D represents a downlink subframe, S represents a special subframe, and U represents an uplink subframe.
TABLE 1TDD uplink-downlink subframe configuration in an LTE systemDownlink-Uplink-to-uplinkdownlinkswitch-subframepointSubframe numberconfigurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
A hybrid automatic repeat request (HARQ) mechanism is used in LTE. For example, in a downlink, after UE receives a PDSCH, if the PDSCH is correctly received, the UE feeds back an acknowledgment (ACK) on a PUCCH. If the PDSCH is not correctly received, the UE feeds back a negative acknowledgment (NACK) on a PUCCH. For FDD, after receiving a PDSCH in a subframe n−4, the UE feeds back an ACK/NACK in a subframe n. For TDD, a time sequence relationship between receiving of a PDSCH and feedback of an ACK/NACK corresponding to the PDSCH is shown in Table 2. In Table 2, numbers 0 to 9 in the first row each represent an uplink subframe n, numbers in the first column represent uplink-downlink subframe configurations, a number k in a column corresponding to a number in the first row constitutes a set K, and the number k indicates that an ACK/NACK corresponding to a PDSCH in a downlink subframe n-k needs to be fed back in the uplink subframe n. For example, in an uplink-downlink configuration 1, when n is 2, K={7, 6}, and it indicates that an uplink subframe 2 is used to feed back an ACK/NACK corresponding to a PDSCH in a downlink subframe n−7 and an ACK/NACK corresponding to a PDSCH in a downlink subframe n−6. The downlink subframe n−7 is a downlink subframe 5, and the downlink subframe n−6 is a downlink subframe 6.
TABLE 2Time sequence relationship between a PDSCH and an ACK/NACKcorresponding to the PDSCH in a TDD systemUplink-downlinksubframeSubframe number nconfiguration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7,——4, 63——7, 6, 116, 55, 4—————4——12, 8, 7, 116, 5, 4, 7——————5——13, 12, 9, 8,———————7, 5, 4, 11, 66——775——77—
LTE further supports a carrier aggregation (CA) technology, that is, a base station configures multiple carriers for UE to increase a data rate of the UE. The multiple carriers are synchronously sent in the time domain, and the UE may separately detect a PDCCH for scheduling each carrier and a PDSCH corresponding to the PDCCH. A specific process of detecting each carrier is similar to that in the foregoing single-carrier case.
The LTE system supports FDD CA, TDD CA, and FDD+TDD CA. TDD CA includes TDD CA with a same uplink-downlink subframe configuration and TDD CA with different uplink-downlink subframe configurations. In a CA mode, one primary component carrier and at least one secondary component carrier are configured for UE, and a PUCCH carrying an ACK/NACK is sent only on the primary component carrier of the UE. A PUCCH sending mode in the CA mode includes a channel selection mode and a PUCCH format 3 mode. In the PUCCH format 3 mode, a discrete Fourier transform-spread-orthogonal frequency division multiplexing (DFT-S-OFDM) transmission scheme is used, a maximum of 20 ACK/NACK bits can be transmitted, and TDD CA of a maximum of five carriers can be supported. For example, in a mainstream TDD uplink-downlink configuration 2 deployed in a current network, an uplink subframe 2 on one carrier can support a feedback of ACK/NACK bits for four downlink subframes, and CA of five carriers with the TDD uplink-downlink configuration 2 supports a feedback of 20 ACK/NACK bits.
With further evolution of an LTE technology, a feedback of more ACK/NACK bits, such as more than 20 bits, may need to be supported in the future. The following describes how to feed back more bits.
For example, CA of more carriers such as CA of ten carriers is introduced. In this case, for example, CA is performed on ten carriers in the TDD uplink-downlink configuration 2, and 40 ACK/NACK bits need to be fed back. For another example, although CA of a maximum of five carriers is supported, a TDD uplink-downlink configuration 5 is configured for most of the carriers. For example, an uplink-downlink configuration 2 is configured for a primary component carrier, and the uplink-downlink configuration 5 is configured for four secondary component carriers. In this case, 4+9×4=40 ACK/NACK bits need to be fed back. However, when an ACK/NACK is fed back by using the current PUCCH format 3, how to reduce overheads when more ACK/NACK bits are fed back is a problem that needs an urgent solution.